DC power supply apparatuses operable from selected one of a plurality of AC voltages are disclosed, for example, in U.S. Pat. No. 5,272,313 issued to K. Karino et al. on Dec. 21, 1993 and in Japanese Examined Patent Publication No. HEI 3-71218 published on Nov. 12, 1991.
According to U.S. Pat. No. 5,272,313, a commercial AC voltage of either 200 V or 400 V can be applied to input terminals of a DC power supply apparatus, and the applied voltage is rectified by an input-side rectifier. The rectified voltage is converted into a high-frequency voltage by two inverters, the inputs of which can be selectively connected in series and in parallel with each other by means of switches. High-frequency voltages from the inverters are rectified by output-side rectifiers. When the voltage supplied to the input-side rectifier is of 200 V, the switches connect the inputs of the two inverters in parallel, and if the voltage supplied to the input-side rectifier is of 400 V, the two inverters are connected in series. Thus, the value of the voltage supplied to each inverter is the same regardless of whether the input voltage is of 200 V or 400 V.
According to Japanese Examined Patent Publication No. HEI 3-71218, a commercial AC voltage of 100 V or 200 V is applied to input terminals of the DC power supply apparatus and is rectified by an input-side rectifier. The rectified voltage is voltage-boosted to a predetermined voltage by a voltage-boosting converter. The boosted voltage is converted to a high-frequency voltage by an inverter, and the high-frequency voltage is voltage transformed by a transformer. The transformed voltage is rectified by an output-side rectifier. Thus, whether the voltage applied to the input-side rectifier is of 100 V or 200 V, the voltage the inverter receives is always the predetermined voltage.
The power supply apparatus according to U.S. Pat. No. 5,272,313 has a disadvantage that it requires two inverters. In addition, currents and voltages to be applied to the two inverters must be balanced with respect to each other, and, therefore, the power supply apparatus requires a balancing circuit arrangement. Thus, the power supply apparatus should be large in size.
According to Japanese Examined Patent Publication No HEI 3-71218, the value of the voltage to be applied to the power supply apparatus is 100 V or 200 V. The value of commercial AC voltages in the world, however, widely ranges. The values of the commercial AC voltages in the world are 200 V, 208 V, 230 V, 240 V, 380 V, 415 V, 440 V, 460 V and 575 V, for example.
In order to use the power supply apparatus disclosed in this Japanese patent publication with such widely ranging voltages, it is necessary to modify it such that the voltage-boosting converter provide, as the predetermined voltage, a voltage having a peak value of the voltage resulting from rectifying the highest voltage of 575 V, i.e. 813 V=575 V.times..sqroot.2. Then, when the lowest voltage, 200 V, is the input voltage, the peak value of the voltage resulting from rectifying it, i.e. 283 V (=200 V.times..sqroot.2) must be boosted up to the predetermined voltage of 813 V. In other words, the peak value must be multiplied by about three. Also, switching devices, such as IGBTs, used in the inverter have to withstand higher voltages so that the power supply apparatus can be used with a voltage of 813 V.
Switching devices which can withstand a voltage of 1200 V are available now. However, even if such high withstand voltage IGBTs are used, the margin of the withstand voltage of IGBTs is narrow relative to the 813 V voltage, and, therefore, the possibility of the IGBTs being damaged is high. In addition, high withstand voltage switching devices are not in the market. High withstand voltage switching devices are inferior in switching characteristic to low withstand voltage switching devices, and are expensive.
Therefore, an object of the present invention is to provide a DC power supply apparatus which is relatively small in size and can use low withstand voltage switching devices for its inverter.